Permafrost cementing process

ABSTRACT

This specification discloses methods of treating wells drilled into the earth. These methods are particularly applicable for cementing behind casing located in wells drilled through permafrost zones. In carrying out these methods, slurries of calcium aluminate cement, clays selected from the group of bentonite, attapulgite and mixtures thereof, and water are formed and placed in the wells to be treated.

United States Patent Inventor Appl. No.

Filed Patented Assignee Joseph U. Messenger Dallas, Tex.

Dec. 9, 1969 June 1, 197 l Mobil Oil Corporation PERMAFROST CEMENTINGPROCESS 13 Claims, 9 Drawing Figs.

US. Cl...

Int. CL... Field of S earch 293, 288;

References Cited UNITED STATES PATENTS Billings Coss Randall SalathielHolmgren et a1.

COMPRESSIVE STRENGTH, psi (24 HRS 32F) 166/288, 166/292 E2lb 33/14166/292, 106/ 104, 64

106/ l 04 106/1 04X 106/ 104 l66/293X 166/293X n 13,5s1,s25

3,197,317 7/1965 Patchen 166/292 3,226,240 12/1965 Crowley 106/104X3,227,213 l/1966 Smith 166/292 OTHER REFERENCES Dier, l. S., New IdeasSolve Permafrost Drilling/Cementing Problems", World Oil May, 1969,pages 89 92.

Anonymous, Gypsum Blend Cements Show Promise for North Slope Use The Oiland Gas Journal May 19, 1969, (page 58). Copy in Group 353.

Primary Examiner-Stephen J. Novosad Attorneys-William J. Scherback,Frederick E. Dumoulin,

William D. Jackson, Henry L. Ehrlich, Andrew L. Gaboriault and Sidney A.Johnson I l l 2 3 4 5 BENTON/TE CONTENT,

wt BASED ON CEMENT PAWNTED JUN Han SHEET 1 [IF 3 FIG,I

Y m E NT. N EN R SE4 0 S T EN T MI A U H w w y J Y. B 8 4 2 G 2 I 2 S RN U mu 0 0 H H E 2cm W M T L H 8 lfi 4 0 55555555 5555 8765432 32 poisssPATENTEDJUN nan T 3.581; 825

sum 3 or 3 TIME, MINUTES o -40 so 120 leo 200 '240 280 rmgumurss JOSEPHu. MESSENGER INVE N TOR ATTORNEY PERMAFROST CEMENTING PROCESS BACKGROUNDOF THE INVENTION This invention relates to calcium aluminate cements andin particular to the use in wells of mixtures of calcium aluminatecement, clays and water.

Oil well cementing practices have been used in completing wells drilledinto the earth since at least the early 1900s and oil well cements havebeen standardized by the American Petroleum Institute (API). In thisstandardization, the term oil well cement" refers to portland cement.

Various additives have been used in portland cements to affect theproperties thereof. For example, in US. Pat. No. 2,582,459 to Richard A.Salathiel, bentonite and a soluble salt of a sulfonic acid are added toportland cement to produce an oil well cement which has low mechanicalstrength and will not shatter under high impact. In US. Pat. No.3,197,317 to Freeman D. Patchen, attapulgite is added to Portland cementslurries for use in oil well cementing to reduce the density of theslurries. In US. Pat. No. 3,227,213 to Dwight K. Smith, bentonite orattapulgite is prehydrated and then mixed with hydraulic cement of thecharacter of portland to form a slurry having a much higher yield than aslurry utilizing dry bentonite.

Calcium aluminate cements have not been in general use for cementingwells. However, they have proved to be ideal for use in thermal recoverywells and in particular when fine silica sand and ground fire brick areused as an admix. They have also been shown by laboratory and fieldexperience to be usable in areas where permafrost problems exist, forexample, on the North Slope of Alaska and Northern Canada. (PetroleumEngineer, Sept. I966, pp. 64-66, High Alumina Cement Solves PermafrostCementing Problems," D. L. Stude) Stude gives data obtained in a modelbuilt to simulate conditions on the North Slope of tests of neat CimentFondu (calcium aluminate cement) and 50:50 mixtures of Ciment Fondu andfly ash which indicate that calcium aluminate cements are to bepreferred over portland cements for use in permafrost zones.

However, certain problems associated with calcium aluminate cements arepointed out by J. S. Dier in New Ideas Solve PermafrostDrilling/cementing Problems," WORLD OIL, May I969; High alumina cementwas not considered when certain specified wells were drilled through thepermafrost because of cost, normal slurry yielding characteristics andtheir sensitivity to contaminants."

As described in US. Pat. No. 3,226,240 to Michael S. Crowley, calciumaluminate cements are prepared by calcining and melting a mixture oflimestone and alumina. For the pure calcium aluminates, the limestone isa chemical grade of at least about 97 percent purity, and the alumina isBayer alumina of at least about 99 percent purity. After calcination,the calcium aluminate is quenched and pulverized. Lumnite and CimentFondu are examples of sintered types which contain alumina up to 43percent (see The Condensed Chemical Dictionary, Sixth Edition, ReinholdPublishing Corporation).

SUMMARY OF THE INVENTION In accordance with the present invention thereare provided new and improved techniques for cementing wells penetratingthe earth. A cement slurry is formed comprising calcium aluminatecement, clay selected from the group consisting of bentonite,attapulgite, or mixtures thereof, wherein the clay is present in anamount no greater than 6 percent by weight based on cement and water inan amount of at least 2.0 gallons per percent of clay in excess of theamount of water required to form a pumpable slurry of calcium aluminatecement, and the slurry-is pumped into the well and allowed to set. In apreferred embodiment of the invention the clay is present in an amountof no greater than 3 percent by weight based on the cement. The clay iscomprised of a mixture of bentonite and attapulgite wherein the ratiosof bentonite to attapulgite vary within the range of 2 to l and 5 to 1.

The invention is particularly useful in cementing behind casing in wellspenetrating a permafrost zone. By employing this technique in such wellsthere is provided a body of set cement in the well formed from a slurryofcalcium aluminate cement and clay selected from the group consistingof attapulgite and mixtures of attapulgite and bentonite, wherein saidclay is present in an amount no greater than 6 percent by weight basedon cement.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of awellbore penetrating a permafrost zone of the earth which illustratesthe placing of a cement slurry in the annulus between the casing and thepermafrost zone.

FIG. 2 is a cross-sectional view of a wellbore penetrating a permafrostzone of the earth which illustrates a cement slurry surrounding thecasing and heat being applied indirectly to the slurry.

FIG. 3 is a plot which illustrates that the heat of hydration of a neatcalcium aluminate cement slurry will maintain the slurry at asufficiently high temperature such that it will set under permafrostconditions.

FIG. 4 is a plot, similar to FIG. 3, which illustrates tailoring theheat of hydration of a calcium aluminate cement slurry by the additionof water and clay thereto.

FIG. 5 is a plot which illustrates the effect upon the com pressivestrength in 24 hours at 32 F. of the addition of various ratios ofbentonite/attapulgite to a calcium aluminate cement slurry containing79.2 weight percent of water based on cement.

FIG. 7 is a plot which illustrates the false set characteristics andthickening times at 40 F. of two calcium aluminate cement slurries, oncontaining 1.5 percent bentonite and the other containing 0.75 percentbentonite and 0.75 percent attapulgite, where each slurry contains 9.4gallons of water per 94-pound sack of cement.

FIG. 8 is a plot which illustrates the effect of various ratios ofattapulgite/bentonite on the false set characteristics of calciumaluminate slurries containing 1.5 percent clay. FIG. 9 is a plot whichillustrates the false set characteristics and thickening times at 40 F.of two calcium aluminate slurries, one containing 1.5 percentprehydrated bentonite and the other containing 0.75 percent prehydratedbentonite and 0.75 percent attapulgite where each slurry contains l0.8gallons of water per 94-pound sack of cement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I have discovered that calciumaluminate cement slurries can be tailored to give improved results inpermafrost zones. This tailoring reduces the rate of hydration ofcalcium aluminate cement to desired rates for use in permafrost zones.Broadly, this tailoring is done by adding to calcium aluminate cementwater in designated amounts and clay selected from the group ofbentonite and attapulgite and mixtures thereof. These slurries thoughparticularly useful under permafrost conditions are also useful fortreating higher temperature wells. However, for simplicity, thisinvention is described by making reference to its use under permafrostconditions.

Severe problems have been encountered in drilling and completing wellsdrilled through frozen formations, referred to as permafrost, found inthe northern areas of Canada and Alaska and particularly along the NorthSlope of Alaska. Permafrost zones are comprised of strata of earth thatexist at subfreezing temperatures generally considered to be within therange of 15 F. to 27 F. The permafrost zones which give the mostproblems in drilling and completing wells are those unconsolidated sandsand gravels which contain ice as the continuous phase of the matrix.Such zones, if thawed, tend to flow or cave away producing excessivehole enlargement. Thus, in permafrost cementing the cement slurry musthave unique properties. Its heat of hydration should be high enough tokeep the slurry from freezing while it sets, but not so high as to meltthe permafrost during the setting period. If the heat of hydration ofthe slurry is too low or if it is released too slowly the slurry freezesand never sets satisfactorily. However, if the heat of hydration of theslurry is too high or is released too rapidly or early in the settingprocess the permafrost is thawed.

For setting well casing through permafrost zones by one embodiment ofthis invention, a slurry is formed of calcium aluminate cement, clayselected from the group consisting of bentonite, attapulgite andmixtures thereof in amounts of no greater than 6 percent by weight basedon cement, and water in an amount of at least 2.0 gallons per percentclay in excess of the amount of water required to form a pumpable slurryof calcium aluminate and the slurry is pumped into the well and allowedto set.

Normally the amount of water that is mixed with cement to form a slurrythereof is expressed in units of gallons per 94- pound sack of cement.The normal amount of water which is required to form a pumpable slurryof clay-free calcium aluminate cement is considered to be 5.2 gallonsper 94-pound sack of cement, though less water can be used if a moreviscous slurry is acceptable and more water can be used if a lessviscous slurry which has poorer settling properties is acceptable. Withthe addition of clay to the cement slurry water in excess of the above5.2 gallons per sack must be used if the viscosity or thickness of theslurry is to be maintained above equivalent to the pumpable clay-freeslurry. I have found that an additional amount of water of at least 2gallons per sack of cement must be used for each 1 percent of clayadded. Thus, if 1 percent of clay or 0.94 pound of clay is added to 94pounds of cement, then 7.2 gallons of water per sack are required ratherthan the 5.2 gallons required for a clay-free sack of cement. Hereafterin this specification and appended claims whenever the amount of waterper percent of clay in excess of the amount of water required to form apumpable slurry of calcium aluminate cement is called for, it is to beunderstood that this means the additional amount of water required perpercent of clay for each 94-pound sack of cement.

Such a slurry of calcium aluminate cement, clay, and water will developsignificant compressive strength in 8 to 12 hours at 32 F. The slurrymay be maintained at 32 F. at the interface between the cement and thepermafrost by tailoring the heat of hydration (primarily by varying theamount of water), controlling the temperature of the mixing water andapplying external heat.

The slurry is then pumped in a conventional manner down the well andpositioned around the casing. For example, the slurry may be pumped downthe well through the casing between two moving plugs, as is illustratedin FIG. I. Referring to FIG. I there is shown a borehole I which hasbeen drilled into the permafrost 3. Casing is positioned within boreholel and extends to approximately the bottom thereof. Guide shoe 7 isattached to the lower end of easing 5, and float collar 9 of locatedwithin casing 5 above guide shoe 7. Cement slurry 11 is shown partiallydisplaced from casing 5 into annulus 13. Lower cement plug 15 is shownwithin casing 5 at float collar 9 and upper cement plug 17 is shownseparating the upper portion of cement slurry 11 from displacementliquid 19 thereabove. Displacement liquid 19 is pumped down casing 5displacing slurry 11 before plug 17 until the movement of plug 17 isstopped by float collar 9. The cement slurry II is then maintainedwithin annulus 13 until it sets and bonds casing 5 to permafrost 3. Aspreviously mentioned, in accordance with the preferred embodiment ofthis invention, cement slurry 11 is mixed at a temperature ofapproximately 40 F. to 45 F. and displaced down casing 5 by displacementliquid 19 which is also at a temperature of about 40 F. to 45 F. By thusplac ing slurry 11 within annulus l3, permafrost 3 is not thawed and thetemperature of slurry 11 will normally be maintained by its heat ofhydration at about 32 F. or at least at a sufficiently high temperatureto set but not at such a high temperature as to cause thawing of thepermafrost 3.

As illustrated in FIG. 2, cement slurry 11 is shown positioned aroundthe lower portion of easing 5 and within the annulus 13 formed betweenthe casing 5 and the wall ofborehole 1. Under some conditions it may bedesirable to apply heat indirectly to cement slurry 11 within annulus 13until cement slurry ll begins developing its heat of hydration. Thisheat may be supplied by circulating heated fluid within casing 5. Forexample, a drill string 21 may be lowered within casing 5 toapproximately the location of float collar 9 and heated fluid 20circulated down drill string 21 and back up annulus 23 formed betweendrill string 21 and easing 5. Generally this heated fluid 20 is at atemperature of about 40 F. to 45 F. However, fluids of othertemperatures, for example, steam, may be circulated in this manner. Theprimary concern in supplying heat indirectly to cement slurry 11 withinannulus I3 is to maintain the slurry at such a temperature, preferablyabout 32 F., so that permafrost 3 is not thawed during the setting ofslurry II. A body of cement bonding easing 5 to permafrost 3 is thusformed.

The fact that the temperature of a neat calcium aluminate cement slurryis maintained by its heat of hydration at a sufficiently hightemperature such that it will set under permafrost conditions is shownby FIG. 3 which illustrates data presented in the aforementioned articleby Stude. In FIG. 3 time in hours is plotted as the abscissa andtemperature in degrees Fahrenheit is plotted as the ordinate. The curveof FIG. 3 is representative of the temperature at the interface of aneat calcium aluminate cement slurry and the permafrost zone as measuredby a thermocouple located at the interface. As shown by the curve, apeak temperature of about 81 F. is reached at this interface. Such ahigh temperature causes undesirable thawing of the permafrost zone inthe vicinity of the well. However, the slurry does set under theseconditions.

I have found that cement slurries comprised of calcium aluminate cement,clay selected from the group consisting of bentonite and attapulgite andmixtures thereof, wherein the clay is present in an amount no greaterthan 6 percent by weight based on cement, and water in an amount of atleast 2.0 gallons per percent of clay per 94-pound sack of cement inexcess of the amount of water required to form a slurry of calciumaluminate cement, will set under permafrost conditions without thawingthe permafrost. Further, the addition of these clays affects the otherproperties of the cement slurry, such as density, settling time,compressive strength, yield, and pumping time in a way which is eitherbeneficial or acceptable for use in the completion of wells. Asatisfactory amount of water to form a pumpable slurry of heat calciumaluminate cement is 5.2 gallons per 94-pound sack of cement. Thus, forexample, if 3 percent clay is used then the amount of water required persack ofcement is at least 5.2+3X2, or I 1.2 gallons.

Data was obtained in a permafrost model in order to clearly show theeffects of tailoring the heat of hydration of a calcium aluminate cementfor use in permafrost zones. FIG. 4 is a plot, similar to FIG. 3, ofthis data and can be compared to FIG. 3. The data of FIG. 4 is obtainedby testing a slurry comprising calcium aluminate cement (Ciment Fondu),attapulgite in an amount of 1 percent by weight based on cement,bentonite in an amount of 2 percent by weight based on cement, and waterin an amount of 80.5 percent by weight based on cement. As shown by thecurve of FIG. 4, the temperature at the interface of the slurry and thepermafrost zone during a time period in which the slurry sets reached amaximum of about 32 F. and remained at that temperature for about 1 lihours after which the temperature began to decrease. This represents anideal condition because the slurry will set in less than I lfzhours at32 F. and the permafrost in the vicinity of the well will not be thawed.

It is to be noted that the above slurry is quite thick and forpracticable purposes is umpumpable. However, FIG. 4 illustrates theeffect of tailoring of the heat of hydration by the addition of watersuch that the temperature of the slurry is maintained at 32 F. duringthe setting of the slurry. A pumpable slurry would be obtained withoutany significant effect upon the heat of hydration of the slurry byreducing the amount of clay so that the water present was equivalent toat least 2.0

gallons per percent of clay in excess of the amount of water required toform a pumpable slurry of calcium aluminate cement.

The amount of water used in the slurries of this invention is determinedas follows. The amount of water required to form a pumpable slurry ofcalcium aluminate cement is taken to be 5.2 gallons per 94-pound sack ofcement. To this is added 2.0 gallons per sack for each 1 percent of claywhich resulting slurry has a consistency of about 30 poises afterminutes of stirring as determined by use of the HalliburtonConsistometer. Thus, 2.0 gallons of water per percent of clay is termedminimum water." The so-called normal amount of water is 2.8 gallons perpercent of clay which when added results in a slurry having aconsistency of about 11 poises after 20 minutes of stirring asdetermined by use of the Halliburton Consistometer.

Thus, the minimum amount of water to be added to form a slurry ofcalcium aluminate cement, 1.125 percent bentonite by weight based oncement, and 0.375 percent attapulgite by weight based on cement isdetermined to be about 73 percent as calculated below:

10 line structure.

I have carried out laboratory tests to demonstrate the applicability ofthis invention for use in cementing wells drilled through permafrostzones. Two trade brands of calcium aluminate cement, Ciment F ondu andLumnite, were used in car- 5 rying out these tests. The results of thesetests are tabulated in TABLE 1 and presented in FIGS. 5 through 9.

FIG. 5 is a plot having an abscissa of bentonite content in weightpercent based on cement and an ordinate of compressive strength inpounds per square inch in 24 hours at 32 F. This FIG. 5 illustrates theeffect of the addition of bentonite to calcium aluminate cement on thecompressive strength which is developed in 24 hours at 32 F. In formingthe calcium alugg gfi minate cement slurries water was used in an amountof 5.2 galcement lons per 94-pound sack of cement plus the normal amountof Amount of water er 94-p d k of water for the various percentages ofbentonite added, which calcium aluminate cementis 8.33 normal amount wasfound to be about 2.8 gallons per percent Pounds /g atBIX5-Z galofbentonite based on a 94-pound sack of cement, or, more Whiter/sackcement 3 precisely, the normal amount of water was found to be 2.8 gal-Mulmum atmount of .Water lons per percent of bentonite for the first 3percent of clay, 2.6

clay (1.120 bentonite, 0.370 attapu1gite is 833 pounds water/gall.gallons for the 4th and 5th percent of bentonite, and 2.2 galwaterx 2 yWater/1% clayx lons for the 6th percent of bentonite. In this test it ISseen that 1.5 clay/sack e e t 25 0 the compressive strength of the setcement containing lxper cent bentonite is about 1,045 p.s.i. and thiscompressive 68.3 pounds water/sack cement 94 pounds cement/sack cement X100=72.7% water is used in preparing the slurry When preparing theslurry for treatment of wells penetrating permafrost zones in accordancewith an embodiment of this invention, mixing water at a temperature ofabout 40 to 45 F. is used and the slurry is displaced down the well bydisplacement liquid also at a temperature of about 40 F. to 45 F. Thisallows the cement slurry to be mixed without incurring problemsassociated with freezing of the water or false setting of the cementslurry. Further, so placing the slurry adjacent the permafrost zone doesnot cause substantial thawing of the permafrost and allows sufficienttime for the slurry to begin developing its heat of hydration whichprevents freezing of the slurry.

strength decreases to about p.s.i. for cement containing 5 percentbentonite. It is generally conceded to be good cementing practice tocement around the lower portion of casing within a well with cementswhich exhibit a compressive strength of at least 500 p.s.i. after 24hours under the condi- 40 tions existing in the well. However, cementshaving less strength than 500 p.s.i. may satisfactorily be used,particularly as lead cements for blocking off waterflows from upperformations. Satisfactory results as a lead cement can be obtained withcements which exhibit compressive strengths of as low as about 5 to 10p.s.i. after 24 hours.

As previously noted, the normal amount of water was used in making thesetests. There is a range of water referred to as minimum water to maximumwater which may be used in mixing cement slurries. When minimum water isused, the resulting set cement exhibits a higher compressive strengththan when either normal or maximum water is used. However, thisincreased compressive strength is attained at a sacrifice of yield(volume of cement slurry per sack of cement). Therefore, the curve ofFIG. 5 would be shifted upward if the ce- TABLE I.-P ROPE RTIES OFCALCIUM ALUMINATE CE MENT SLURRIES CONTAINING GEL AND/OR ATTAPULGITEConsis- Halllburton Mixing water Comprestency thickening sive after 20time at 40 F.

weight Sack strength, min, percent Slurry Yield, weight p 5.1. in HalliWater Min. to Min. to on Gall density, on. it./ 1b.] 24 hrs. burtonseparastart 70 Cement formulation used cement sack 1b./gal sack sack at32 F. unit-s tion, ec. of set poises Lumnite 46 Lumnite:

Plus 1% gel 69. 2 7.81 14. 0 1. 53 Plus 2% gel 95. 8 10. 81 12. 85 1. 93Plus 3% gel 120 13. 12. 1 2. 32 Plus 4% gel 141. 5 16.00 12.2 2. Plus 1%attapulgite 69. 2 7.81 14.0 1. 53 Plus 2% gel and 1% attapulgite 124. 014. 00 12. 0 2, 37 Plus 2% gel 95. 8 10.81 12. 85 1. 93 Plus 1% gel and1% attapulgite 95. 8 10.81 12.85 1. 93 Pills 2% attapulgite 95. 8 10. 8112. 85 1.93 Ciment Fondu:

Plus 1.5% gel 79. 2 9. 40 13. 25 1.

Plus 2.0% gel 95.8 10. 81 12.85 1. 93 Plus 3.0% gel. 1'20 13. 60 12.1 2.32 Plus 4.0% geL. 142 16.00 11.7 2. 64 P1115 50% gel 163 18. 40 11. 352.97

TABLE 1. 1RO1ERT1ES()FlfALL'll'MALLMINATE CEMENT SLI'RRIES CONTAININGGEL AND/R ATTAIL'LGITE Consis- I-Ialliburtou Mixing water Comprestencythickening sive after 20 time at 40 F.

Weight Sack strength, min. Water percent Slurry Yield. weight p.s.i. inIlallisepara- Min. to Min. to on Gal./ density, cu. it./ 1b.] 24 hrs.burtpn tion, cc. start 7O Cement fomiulation used cement sack lbJgal.suck suck at 32 F. units of set poises 1, 121 Plus 0.125% attapulgiteand 1.375% gel 7s. 2 9. 40 13.25 1. 75 1.220 1o 1 1,1 1 1. 517 1, 233Plus 0.25% attapulgite and 1.25% gel 79. 2 9. 40 13. 25 1. 75 98g 10 1,07 3 1, 203 1, 254 Plus 0.375% attapulgite and 1.125% gel 79. z 9. 4013. 75 2 1, 1 7g 1, 1 Plus .50? attapulgite and 1.00? 1101 70.1 0. 4013. 25 1.75 1,187 4 Plus 0.75% attapulgite and 0.75% gel... 711.2 0. 4013. 25 1.75 1,130 5 Plus 1.00? attapulgite and 0.500} gel.. 7. L 0.4013. 25 1.7 1,130 6 Plus 1.25% attapulgite and 0.259} 111. T0. 2 1 .4013.25 1. 75 045 4 Plus 1.0% attapulgite and 10% gel. 95. 8 10.81 12.85 1. 93 Plus 1.5% attapulgite 79. 2 9. 40 13.25 1.75 051 Plus 2.0%attapulgite 95. 8 10.81 12. 85 1. 03 Plus 2% prehydrated gel 95. 8 10.811'1. 85 1. 93 Plus 1% prchydratcd gel and 1% attapulgite. 95. 8 10.8112. 85 1. 93

1 3 days at 32 F. Average.

ment had been mixed with minimum water and would be shifted downward ifthe cement had been mixed with maximum water. Likewise, a similar curvewould be obtained if attapulgite had been used instead of bentonite.However, the attapulgite curve would be shifted downward since theaddition of equivalent amounts of attapulgite rather than bentoniteproduces a set cement having less compressive strength. This isillustrated by FIG. 6 described below. Thus, it is seen that when clayselected from the group consisting of bentonite and attapulgite, andmixtures thereof, is added to calcium aluminate cement in an amount nogreater than 6 percent by weight based on cement and allowed to set, acement is formed having compressive strength which is satisfactory foroil well use. The upper ranges of the amount of clays added to thecalcium aluminate cement are nonnally utilized when preparing a leadcement. Preferably, no greater amount than 3 percent by weight based oncement of bentonite or attapulgite or mixtures thereof are added tocalcium aluminate cements, when the slurry is to be used for cementingaround the lower portion of the casing to hold the casing in place. Whenthese preferred amounts of not greater than 3 percent clay are added, aset cement is formed which exhibits a compressive strength of about 500p.s.i. or greater in 24 hours at 32 F. which is completely satisfactoryfor permafrost use.

FIG. 6 illustrates the effect of the addition of attapulgite andbentonite and mixtures thereof to calcium aluminate cement on thecompressive strength in 24 hours at 32 F. In this figure various ratiosof bentonite and attapulgite wherein the total amount of both is 1.5weight percent based on cement are plotted as the abscissa andcompressive strength, pounds per square inch in 24 hours aft 32F, isplotted as the ordinate. In this test, 1.50 weight percent clay based oncement was added to calcium aluminate cement. The addition of 1.50percent attapulgite to calcium aluminate cement resulted in a set cementhaving a compressive strength of about 950 p.s.i. and the addition of1.50 percent bentonite to the calcium aluminate cement resulted in a setcement having a compressive strength of about 1,045 p.s.i. Thecompressive strengths of set cements containing different ratios ofattapulgite and bentonite vary, generally increasing from a minumumstrength of 950 p.s.i. when attapulgite alone is used to a maximumstrength of 1,215 p.s.i. when a mixture of 1.215 percent bentonite and0.375 percent attapulgite are used. Thereafter the strength drops off to1,045 p.s.i. when bentonite alone is used. Thus, in accordance with apreferred embodiment of this invention, the preferred amount of clay tobe used in forming a calcium aluminate cement slurry varies between 1.00percent bentonite to 0.50 percent attapulgite and 1.25 percent bentoniteto 0.25 percent attapulgite or in other words between the ratios of 2:1and 5:1 bentonite to attapulgite.

FIG. 7 is a presentation of data which illustrates the pumping and falseset characteristics at 40 F. of two calcium aluminate cement slurries;one, curve A, containing 1.5 percent bentonite, and the other, curve B,containing 0.75 percent bentonite and 0.75 percent attapulgite. In thisplot the abscissa is time in minutes in a Halliburton Consistometer at40 F. and each ordinate is Halliburton Consistometer poises. The falseset characteristics are identified by the initial part of the curve asindicated by D of curve A. For example, a slurry of Ciment Fondu plus1.5 percent bentonite, curve A, initially gives a consistency of 11poises which almost immediately increases to 15 poises and thendecreases to about 7 poises after minutes and does not increase again to11 poises until about 88 minutes. Thus, the slurry offers a highconsistency initially and thereafter decreases to a minimum value beforebeginning to increase again as the slurry thickens with time. This highinitial consistency D, is what is referred to as false set and can undersome conditions get so high that the slurry cannot be pumped even thoughthe slurry has not set. The calcium aluminate cement slurry containing amixture of bentonite and attapulgite, curve B, exhibits only minimal orno false set characteristics. The initial consistency is about 4 poisesand remains essentially constant for about the first minutes at whichtime it begins increasing, but at a slower rate than shown by curve A.Thus, the pumping time for a calcium aluminate cement slurry containing1.5 percent bentonite at 40 F. is considered to be about 88 minuteswhereas the pumping time of the slurry containing 0.75 percent bentoniteand 0.75 percent attapulgite is considered to be 115 to 148 minutes.These pumping times are based upon the API mixing time which is the timerequired to reach 70 poises. It is seen that by blending the attapulgiteand bentonite together the false set characteristics shown whenbentonite alone is used are decreased and the pumping time is increased.

This reduction in the false set characteristics of calcium aluminatecement and bentonite produced by blending attapulgite with bentonite isfurther illustrated in FIG. 8, which is another plot similar to FIG. 7.Here a total of 1.5 percent clay is used but the ratios of attapulgiteto bentonite vary. The slurry of curve F contains 0.125 percentattapulgite and 1.375 percent bentonite and shows a false setcharacteristic J. The slurry of curve G contains 0.25 percentattapulgite and 1.25 percent bentonite and shows a false setcharacteristic K which is less than J of curve F. Likewise, the slurryof curve H has 0.375 percent attapulgite and 1.125 percent bentonite andshows false set characteristic L which is less than K of curve G. Theslurry of curve 1 contains 0.75 percent attapulgite and 0.75 percentbentonite and for all practicable purposes can be considered to show nofalse set characteristic. Thus, FIG. 8.

clearly illustrates the improvement in false set characteristics of acalcium aluminate cement slurry which results from increasing the amountof attapulgite as compared to bentonite used in the slurry. A slurrycontaining either large amounts of attapulgite as compared to bentoniteor attapulgite alone exhibits only minimal or no false setcharacteristics. This is highly desirable when the slurry is to be usedunder certain ce menting conditions.

FIG. 9 is another plot similar to H68. 7 and 8 but where prehydratedbentonite rather than dry bentonite is used. Curve P is a plot of dataobtained by testing calcium aluminate cement containing 1.5 percentprehydrated bentonite, and

curve Q is a plot of data obtained by testing calcium aluminate cementcontaining 0.75 percent prehydrated bentonite and 0.75 percentattapulgite. The prehydrated bentonite shows false set characteristics Rsimilar to dry bentonite. Also, the yield of prehydrated bentonite incalcium aluminate cement is about the same as the yield of dry bentonitein calcium aluminate cement, which is a vast distinction from Portlandcement slurries where approximately 4 times as much dry bentonite mustbe used as prehydrated bentonite to obtain the same yield.

What I claim is:

1. In a process of treating a well penetrating the earth, the stepscomprising:

a. forming a cement slurry comprising calcium aluminate cement, clayselected from the group consisting of bentonite and attapulgite andmixtures thereof in an amount of no greater than 6 percent by weightbased on cement, and water in an amount of at least 2.0 gallons per each1 percent of clay in excess of the amount of water required to form apumpable slurry of clay-free calcium aluminate cement;

b. introducing said cement slurry into said well; and

c. allowing said cement slurry to set within said well.

2. The process of claim 1 wherein water is present in an amount of about2.8 gallons per each 1 percent of clay in excess of the amount of waterrequired to form a pumpable slurry of calcium aluminate cement.

3. The process of claim 1 wherein said clay is present in an amount nogreater than 3 percent by weight based on cement.

4. The process of claim 1 wherein said clay is bentonite in an amount nogreater than 6 percent by weight based on cement.

5. The process of claim 4 wherein said bentonite is present in an amountno greater than 3 percent by weight based on cement.

6. The process of claim 1 wherein said clay is attapulgite in an amountno greater than 6 percent be weight based on cement.

7. The process of claim 6 wherein said attapulgite is present in anamount no greater than 3 percent by weight based on cement.

8. In a process of treating a well penetrating a permafrost zone of theearth, the steps comprising:

a. forming a cement slurry comprising a mixture of calcium aluminatecement, clay selected from the group consisting of bentonite andattapulgite and mixtures thereof wherein said clay is present in anamount of no greater than 6 percent by weight based on cement, and waterin an amount of at least 2.0 gallons per each 1 percent of clay inexcess of the amount of water required to form a pumpable slurry ofcalcium aluminate cement;

b. positioning said slurry adjacent said permafrost zone; and

c. allowing said slurry to set.

9. The process of claim 8 wherein said clay is a mixture of bentonitepresent in an amount of 1.125 percent by weight based on cement andattapulgite present in an amount of 0.375 percent by weight based oncement.

10. The process of claim 8 wherein the amounts of bentonite andattapulgite vary between the ratios of 2:1 and 5:1 bentonite toattapulgite.

11. In a well equipped with casing and penetrating a permafrost zone ofthe earth, a body of set cement surrounding said casing within saidpermafrost zone, said set cement being comprised of calcium aluminatecement and clay selected from the group consisting of attapulgite andmixtures of attapulgite and bentonite, wherein said clay is present inan amount of no greater than 6 percent by weight based on cement.

12. In a process of treating a well equipped with casing penetrating apermafrost zone of the earth, the steps comprising:

a. fonning a cement slurry comprising calcium aluminate cement, clayselected from the group comprising bentonite and attapulgite andmixtures thereof, wherein said clay is present in an amount of nogreater than 6 percent by weight based on cement, and water of atemperature of about 40 F. to 45 F. in an amount of at least 2.0 gallonsper each 1 percent of clay in excess of the amount of water required toform a pumpable slurry of calcium aluminate cement;

b. pumping said slurry down said well;

c. displacing said slurry from said well with a displacing liquid at atemperature of about 40 F. to 45 F. wherein said slurry is positionedintermediate said casing and said permafrost zone; and

(1. allowing said slurry to set therein.

13. The process of claim 12 further comprising prior to step (d)circulating fluid at a temperature greater than 32 F. through said wellin indirect heat exchange with said cement slurry.

2. The process of claim 1 wherein water is present in an amount of about2.8 gallons per each 1 percent of clay in excess of the amount of waterrequired to form a pumpable slurry of calcium aluminate cement.
 3. Theprocess of claim 1 wherein said clay is present in an amount no greaterthan 3 percent by weight based on cement.
 4. The process of claim 1wherein said clay is bentonite in an amount no greater than 6 percent byweight based on cement.
 5. The process of claim 4 wherein said bentoniteis present in an amount no greater than 3 percent by weight based oncement.
 6. The process of claim 1 wherein said clay is attapulgite in anamount no greater than 6 percent be weight based on cement.
 7. Theprocess of claim 6 wherein said attapulgite is present in an amount nogreater than 3 percent by weight based on cement.
 8. In a process oftreating a well penetrating a permafrost zone of the earth, the stepscomprising: a. forming a cement slurry comprising a mixture of calciumaluminate cement, clay selected from the group consisting of bentoniteand attapulgite and mixtures thereof wherein said clay is present in anamount of no greater than 6 percent by weight based on cement, and waterin an amount of at least 2.0 gallons per each 1 percent of clay inexcess of the amount of water required to form a pumpable slurry ofcalcium aluminate cement; b. positioning said slurry adjacent saidpermafrost zone; and c. allowing said slurry to set.
 9. The process ofclaim 8 wherein said clay is a mixture of bentonite present in an amountof 1.125 percent by weight based on cement and attapulgite present in anamount of 0.375 percent by weight based on cement.
 10. The process ofclaim 8 wherein the amounts of bentonite and attapulgite vary betweenthe ratios of 2:1 and 5:1 bentonite to attapulgite.
 11. In a wellequipped with casing and penetrating a permafrost zone of the earth, abody of set cement surrounding said casing within said permafrost zone,said set cement being comprised of calcium aluminate cement and clayselected from the group consisting of attapulgite and mixtures ofattapulgite and bentonite, wherein said clay is present in an amount ofno greater than 6 percent by weight based on cement.
 12. In a process oftreating a well equipped with casing penetrating a permafrost zone ofthe earth, the steps comprising: a. forming a cement slurry comprisingcalcium aluminate cement, clay selected from the group comprisingbentonite and attapulgite and mixtures thereof, wherein said clay ispresent in an amount of no greater than 6 percent by weight based oncement, and water of a temperature of about 40* F. to 45* F. in anamount of at least 2.0 gallons per each 1 percent of clay in excess ofthe amount of water required to form a pumpable slurry of calciumaluminate cement; b. pumping said slurry down said well; c. displacingsaid slurry from said well with a displacing liquid at a temperature ofabout 40* F. to 45* F. wherein said slurry is positioned intermediatesaid casinG and said permafrost zone; and d. allowing said slurry to settherein.
 13. The process of claim 12 further comprising prior to step(d) circulating fluid at a temperature greater than 32* F. through saidwell in indirect heat exchange with said cement slurry.